Portable terminals, such as mobile phones, Personal Digital Assistants (PDAs) and notebook computers, and desktop Personal Computers (PCs) are frequently utilized in public places. At this time, contents on display monitors can be viewed by all the persons located within visible distances from the display monitors. Due to such a security problem, when a computer is used for the writing of text, mail, chatting or video watching and a user does not want other persons to view the contents thereof, the use of a computer is limited. A privacy problem may arise not only when computers are privately used but also when confidential documents are written using computers in corporations or government offices. Besides, the security problem exists in various fields. For example, Automatic Teller Machines (hereinafter referred to as “ATMs”) are deployed in public places, so that security information, such as the key inputs of passwords of ATM users and transaction details on screens, can be easily exposed. Accordingly, it is useful to develop a display that provides private information to an authorized user on a public monitor and prevents unauthorized persons from viewing the private information on the same public monitor.
Early Liquid Crystal Display (hereinafter referred to as “LCD”) monitors have narrow viewing angles, so that screens look dark when viewed from locations offset from locations in front of the screens. Since the above-described feature is inconvenient in terms of general use, technological efforts have been made to widen the viewing angle thereof. However, the above-described feature is somewhat advantageous in terms of security, and a LCD monitor having the above-described feature may be regarded as the earliest private display. A display having micro blocking members developed from such a concept is disclosed in U.S. Pat. No. 5,528,319 (invented by Russel). However, this scheme is defective in that contents on a display are completely exposed to persons located behind a user. Sceptre Co. in 1998. This product using polarization is constructed so that a polarizing plate is eliminated from the interior of a typical LCD monitor and then a user views the security monitor using polarizing glasses. However, this product is inconvenient for general use because a user can only view the contents when he wears the polarizing glasses, even in circumstances in which privacy is not important. Furthermore, an unauthorized person wearing glasses having simple polarizing characteristics, such as typical sunglasses, can view the contents, so that security is low. As a result, this product failed in the market, and the manufacture of this product was stopped. MMI Co. (http://www.man-machine.com/invisivw.htm) somewhat improved this technology and commercially sold a product, to and from which a polarizing plate may be selectively attached and detached, in 2001. However, this product is still defective in that an unauthorized person wearing simple polarizing glasses, such as sunglasses, can view contents.
The most perfect private display is a Head-Mounted Display (hereinafter referred to as an “HMD”). However, the HMD is expensive, heavy to wear and power-consuming because both a display and an optical system are accommodated in glasses.
For displays that are not private displays but technologically related displays, multi-screen displays, in which a single display shows two different types of images, and users having shutter glasses view their own images, respectively, are disclosed in Korean Pat. Appl. No. 1991-0000391 (filed by Samsung Electronic Co. and entitled “multiple-screen display device and viewing device for monitors”), Korean Pat. Appl. No. 1997-04686 (filed by Samsung Electronic Co. and entitled “video apparatus for simultaneous viewing of two screens”) and Korean Pat. Appl. No. 1999-0051191 (filed by Hoyseung Choi and entitled “apparatus for simultaneous reproduction of multi-type images”). The technology, in which the multi-screen display is applied to a game, is disclosed in U.S. Pat. No. 5,963,371 (assigned to Intel Corporation). The multi-screen displays cannot be considered to be private displays because persons other than persons wearing the shutter glasses can view some of the contents displayed on the displays. That is, the shutter glasses are used only to block other images, and there is no provision for a means for protecting private images.
In the present specification, a single display screen distinguished by the vertical sync of a monitor is referred to as a monitor frame, and a section of image data is referred to as an image data frame. The size of a single image data frame may be identical with or different from that of a single monitor frame. A private image (hereinafter referred to as a “P image”) is the private, non-public image of an authorized user. A masking image (hereinafter referred to as an “M image”) is an image that blocks the P image of an authorized user.
A private display for protecting private information using shutter glasses is currently being implemented. Since this private display is inexpensive, is light to wear and can be developed further, this scheme is regarded as the most competitive method currently. The private display should fulfill all three performance conditions, including ‘user visual perception performance,’ ‘naked eye security performance’ and ‘anti-peeper security performance.’ The ‘user visual perception performance’ is the performance that allows an authorized user to clearly view an image without visual inconvenience or strain, the ‘naked eye security performance’ is the performance that prevents unauthorized persons having no shutter from clearly viewing an image, and the ‘anti-peeper security performance’ is the performance that prevents unauthorized persons or intentional peepers with a shutter from clearly viewing an image.
In the present specification, a shutter opening/closing sequence state value (state information) is the value indicating the extent of opening/closing, and a shutter opening/closing sequence is the sequence of state values corresponding to an image sequence and represented like [0, 1, 0, 1, 0, . . . ]. A shutter opening/closing signal is the signal that is transmitted to control the opening/closing operations of a shutter in accordance with the shutter opening/closing sequence, and generally includes one or more of shutter opening/closing sequence state values.
A P/M image sequence scheme (hereinafter referred to as a “Sun Microsystems' scheme”) disclosed in U.S. Pat. No. 5,629,984 by Sun Microsystems, Inc. is illustrated in FIGS. 1a and 1b. This scheme is one of synchronous schemes, which opens/closes shutter glasses while alternately displaying a private image data frame P and a masking image data frame M in accordance with vertical sync Vsync that is the frame sync of a monitor, thus allowing only a user possessing the shutter glasses to view private images. The basic alternation of the P and M image frames is that the P and M image frames are displayed one after another, as shown in FIG. 1a. Alternately, as shown in FIG. 1b, alternation having a ratio of 1:m (m=1, 2, . . . ), in which, whenever the P image frame is displayed one time, the M image frame is displayed m times, has been proposed. The sequence of U.S. Pat. No. 5,629,984, in which P and M images are alternated with each other at a ratio of 1:1, has weak ‘anti-peeper security performance’ because the sequence can be easily interpreted when a peeper learns a refresh rate and conforms to vertical sync. Furthermore, since only a white flash image is generated as a masking image, it is difficult to conceal a private image. Meanwhile, the sequence, in which P and M images are alternated with each other at a ratio of 1:m (when m is larger than 2), fulfills ‘naked eye security performance.’ However, as m becomes larger, the number of private images becomes smaller and the difference between an opening duration and a closing duration becomes greater, so that ‘user visual perception performance’ is degraded. Furthermore, since the alternation sequence of 1:m is a periodic sequence, a peeper can learn m by learning a refresh rate, conforming to a vertical sync and performing scanning, the sequence can be learned without difficulty, so that the alternation sequence of 1:m has weak ‘anti-peeper security performance.’ Additionally, in the patent, a shutter opening/closing signal is transmitted without being encrypted at every time of opening/closing, so that it is easy for a peeper to intercept the shutter opening/closing signal.
A P/M image sequence scheme (hereinafter referred to as a “IBM's scheme”) disclosed in GB Unexamined Pat. Publication No. 2360414 A is illustrated in FIG. 1c. This scheme is one of asynchronous schemes, which transmits a private image data frame P and a masking image data frame M to a monitor in accordance with Data sync Dsync. The data sync Dsync is not in accordance with monitor frame sync Vsync, image data frames are asynchronously displayed, and data sync Dsync is synchronized with shutter glasses. In this scheme, the display periods Dcycle of the P and M image frames are varied and thus encrypted, so that ‘anti-peeper security performance’ is improved. The shutter opening/closing signal is encrypted by binding a plurality of shutter opening/closing sequences for a certain period and a plurality of image frame display periods, and the encrypted shutter opening/closing signal is transmitted to a shutter. The shutter decodes the encrypted shutter opening/closing signal, and then synchronizes the decrypted shutter opening/closing sequences and image frame display periods with the data sync of image frames using the timer of the shutter. Accordingly, ‘anti-peeper security performance’ is increased for a peeper to intercept the shutter opening/closing signal. The density of light intensity varies with the regions of the monitor due to the asynchronization between an image frame and a monitor frame, so that it is inconvenient for a user to view images. The non-uniformity of an image occurs in a boundary region where the P image frame is alternated with the M image frame due to the combination of an asynchronization with the finite response time of the shutter, so it is inconvenient for a user to view images. Furthermore, the probability that a P image frame is displayed on a specific region (upper end portion) of a monitor is increased, so that ‘naked eye security performance’ and ‘anti-peeper security performance’ are decreased. Even in an asynchronous scheme, an image frame repeatedly alternates between a P image frame and an M image frame, so that the probability that a peeper views a private image through tuning becomes higher. The manufacturing costs of the shutter and power consumption are increased in that the timer is required at a receiving side for the transmission of the encrypted shutter opening/closing signal and synchronization. The masking image data of an M image frame is generated as a simple random pattern image, so that the present scheme is not effective in concealing a private image.
The private display of Mitsubishi Electric Research Laboratory (MERL) disclosed in February of 2002 at a website (http://www.merl.com/papers/TR2002-11/) is based on a synchronous scheme. The private display of MERL generates the inverse image of private image data as masking image data using the time integration of the eye and displays the inverse image, so that unauthorized person is made to view a uniform gray image, which is the mean image of the private image and the inverse image, thereby improving ‘naked eye security performance.’ In order to improve ‘anti-peeper security performance,’ a P/M image frame sequence is randomly generated and provided. In particular, using a high-price fast shutter such as Ferroelectric Liquid Crystal (FLC), shuttering based on frames rather than pixels has been proposed. In the MERL's scheme, when a reverse image is generated, the gamma of a monitor is taken into consideration. However, the understanding of human visual perception is insufficient, so that an incomplete reverse image is generated. Furthermore, a long time is required to calculate a reverse image per frame, so that a real-time system cannot be implemented with the MERL's scheme.
In the MERL's scheme, when a reverse image is generated, the gamma of a monitor is taken into consideration. Furthermore, since a long period is required to calculate a reverse image per frame, a real-time system cannot be implemented with the MERL's scheme, so that the MERL's scheme is applied only to a private display for still images. Furthermore, in the private display of MERL, a disturbing image having a cognitive meaning is used as a masking image. In this case, by reducing the dynamic range of a private image compared to a masking image, a disturbing image is made to be more clearly viewed. In this case, the dynamic range is the concept identical with the difference between the maximum and minimum values of a color space, the range of monitor brightness, and the range of voltage applied to a monitor. However, in the MERL's scheme, a particular photo image, which is effective in concealing a particular still private image, is used as a disturbing image, so that the MERL's scheme cannot be a systematic and strategic disturbing image generating method and does not take human visual perception characteristics into consideration. Furthermore, the MERL's scheme does not present a method of providing a disturbing image per frame in real time, so that the MERL's scheme can be applied only to a private display for still images.